Variable voltage power supply circuits frequently utilize SCR's and triacs which are triggered at selected times during a power signal cycle. The power transmitted through the SCR or triac is dependent on the portion of the power supply cycle which is transmitted through the device. The time of triggering the device is typically based upon the zero crossover as a reference. Therefore, the accuracy of the voltage control is substantially dependent upon the determination of the time of zero crossover. However, power signals frequently carry substantial noise interference and this interference can disrupt the determination of the zero crossover. Interference can cause early or late triggering, thereby making the voltage produced by the power supply greater or less than that desired. The power signal can be filtered to eliminate high frequency interference but the filtering process introduces substantial phase error which can further add to the difficulty in determining the true zero crossover.
In view of the problems of noise and phase shift encountered in the triggering of SCR type power components and the need to produce accurate voltages, particularly in high voltage applications, there exists a need for a power supply circuit for accurately and consistently determining the zero crossover of a power signal which can be subjected to substantial noise interference.